Coronary artery disease is the most prevalent medical problem in the industrialized world. It accounts for over 40% of all deaths in the United States and Western Europe (1). A primary therapy for coronary artery disease is coronary angioplasty with stent implantation (2).
Until recently the success of coronary stenting was limited by the process of restenosis, which occurred in 20-40% of cases (3). There have been reports that risk of developing in-stent restenosis appears to depend upon both clinical and procedural factors such as diabetes, length of lesion and location of lesion (4, 5, 6). The clinical and lesion related risk factors for restenosis, however, are very poorly predictive of restenosis. The most recent approach to prevention of restenosis involves local drug delivery to the vessel wall using drug eluting stents (7, 8). Remarkable success has been achieved in reducing restenosis with the use of stents coated with either Sirolimus or Taxol (9).
Coronary angioplasty with placement of one or more drug eluting stents has become the most common interventional treatment of significant flow limiting coronary artery disease. However, only 2-3 of every 10 patients can be expected to derive an additional benefit beyond that conferred by bare metal stents (10). Bare metal stents are utilized much less frequently compared to drug eluting stents because restenosis is a known problem associated with using bare metal stents. Using a drug-coated stent instead of a bare metal stent is associated with excess costs. Also, although drug-eluting stents would appear to be a significant step forward in the treatment of coronary artery disease, there is concern regarding the long term risk of sub-acute thrombosis associated with drug eluting stents (11). This may occur at an annual rate of 0.2-0.6% and may result in significant excess morbidity and mortality (11). Of note, seventy to eighty percent of patients derive no clinical benefit from drug-eluting stents (10). Therefore, it would be of value to have a test that would be predictive of low risk of restenosis with bare metal stents thereby reducing the indiscriminate use of drug eluting stents with their associated long term risks.
Despite the length of time that restenosis has been an issue and has been the subject of research no current technology exists for reliably determining whether an individual patient is likely to experience restenosis if given a bare metal stent rather than a drug coated stent, i.e., whether the drug coated stent is truly required to prevent restenosis. In addition to the excess costs associated with drug eluting stents, other associated clinical problems may be introduced by indiscriminately using drug coated stents for some patients who might have similar outcomes with a bare metal stent. These include an excess risk of bleeding associated with the need for long term anti-platelet agents in those individuals receiving drug elution stents and the risk of sub-acute stent thrombosis, a particularly severe adverse event associated with 40% mortality rate, in those individuals who may be non-compliant with the prescribed regimen or required to stop anti-platelet agents for other non-related surgical procedures. A method of identifying individual patients who may be equally well treated with a bare metal stent compared to a drug coated stent, from the perspective of whether restenosis is a risk for the patient, would be highly desirable. A gene expression test that accurately predicts bare metal stent restenosis would reduce the need for drug eluting stents and thus reduce long term excess morbidity, costs and mortality associated with drug eluting stents. In light of the vast numbers of patients receiving drug coated stents on a world wide basis, the introduction of simple, rapid and highly predictive gene expression test for prediction of the need for a drug coated stent would have significant beneficial public health consequences.
It has been suggested that restenosis of bare metal stents is due to individual genes. Walter et al (12) reported on 650 consecutive patients receiving non-drug eluting coronary stents and demonstrated a significant association of platelet glycoprotein IIIa gene polymorphism, risk of restenosis and statin therapy. Carriers of the P1A2 allele (A1/A2 heterozygotes) demonstrated a significantly increased restenosis rate, and this difference was largely eliminated by the addition of statin therapy. Walter et al concluded that, “statins interfere with the functional consequence of a genetically determined platelet mediated risk factor”. Bauters et al. (13) presented data which supported the involvement of an I/D polymorphism of angiotensin-converting enzyme as a risk factor for coronary restenosis. Their results, however, were not confirmed by Momotte et al (14). de Maat et al. (15) suggested that a common promoter variant of the human stromolysin-1 gene confers a genotype specific response to medication in determining clinical event free survival and the risk for clinical restenosis after angioplasty and, therefore, may serve as a predictor of clinical restenosis. Specifically, patients with the 5A6A or 6A6A variant of the promoter region of this gene had significantly fewer events when treated with the lipid lowering drug pravastatin compared to placebo treated patients. Those with the 5A5A variant did not experience event reduction regardless of treatment assignment. Kastrati et al. (16) demonstrated that the presence of allele 2 of the interleukin receptor antagonist gene is associated with a lower risk of both angiographic and clinical restenosis. A clear gene dose effect was noted as patients homozygous for allele 2 had a lower incidence of in-stent restenosis than heterozygotes. Zohlnhofer et al. (17) demonstrated up-regulation of FK506-binding protein-12 (FKBP12) in neointimal specimens obtained by atherectomy post angioplasty. This is of interest in that FKBP12 is a target (receptor) for Sirolimus, an agent recently found to significantly reduce restenosis when applied to coronary stents. None of the above-mentioned studies of single-genes have been replicated or confirmed.
The expression of an individual gene may not be reliably associated with an outcome. It has been observed that, for the same disease, with similar inclusion and exclusion criteria, and employing the same gene array platform, multiple studies provide dissimilar sets of differentially expressed genes. The reasons for the unreliability of the association between an individual gene and an outcome include: 1) intra-patient and inter-patient variability; 2) small patient sample sizes; 3) low predictive power of most genes; 4) gene microarray platform variance due to their extremely high sensitivity to noise and 5) to bias in sample collection, handling, and pre-processing; 6) variation in processing reagent batch, decay in reagents over time 7) to differences in reading chip wells and 8) in multivariate analysis, the correlation of individual genes.
The prior art has thus far failed to provide a reliable method for assessing the risk of restenosis in individual patients slated for stent placement.